Fiber-Optic Sensing: A Historical Perspective - qXwave

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CULSHAW AND KERSEY: FIBER-OPTIC SENSING: A HISTORICAL PERSPECTIVE 1071 Fig. 15. Loss-based distributed sensors using (a) microbend and (b) cladding loss modulation mechanisms. Fig. 16. Basic mechanisms of Raman and stimulated Brillouin scatter and typical stimulated Brillouin frequency shifts (lower). in the upshifted and downshifted directions produces a ratio which is uniquely related to temperature. This relationship has been used extensively in distributed temperature probes. Brillouin scatter is a related phenomenon but the energy differentials concerned reflect the acoustic phonon spectrum rather than the optical phonon spectrum. Here, stimulated Brillouin scatter is especially interesting. In stimulated Brillouin, backscattered radiation couples exactly to an acoustic wave whose wavelength is exactly half that of the incoming light. The coupled wave is a frequency shifted by the corresponding acoustic frequency and measuring this frequency shift together with knowing the acoustic wavelength (that is the optical wavelength) immediately gives acoustic velocity along the core of the fiber. This, in turn, depends upon the stiffness:density ratio, dominated by stiffness variations. These, in turn, depend on temperature and strain. Stimulated Brillouin scatter can, therefore, be used to detect varying strain fields given sufficient background knowledge of any temperature variations (Fig. 16).
1072 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 9, MAY 1, 2008 Both Brillouin and Raman scatter have the benefit that they do not involve either measuring or modulating the optical loss from the fiber. Consequently, both mechanisms are viable over extremely long interrogation distances, up to many tens of kilometers. Couple this with the ability to examine spatial increments of the order of one meter (or less with sufficient processing) combined with temperature resolutions of the order of 1 C and strain resolutions measured in the microstrains, we have an immensely power tool. The simplest is Raman scatter and the so called DTS (distributed temperature sensors) has made significant inroads into fire alarm systems in tunnels, overheat alarms in electrical machinery, for example escalators in underground systems and a wide variety of similar applications. Brillouin scatter systems have also found application in similar temperature measuring requirements though the signal processing can be more complex and, therefore, more costly than the Raman equivalent. Brillouin systems come into their own in strain measurement and have found application in monitoring strain fields on railroad tracks where electromagnetic radiation is a significant issue and in measuring the performance of overhead power lines. Distributed sensing will expand its acceptance in the coming years as its unique and powerful capabilities become more widely known with applications in environmental monitoring, safety, and security systems, marine and aerospace structures, civil engineering, and many other sectors. All these applications will require slightly different packaging solutions for the sensor elements (which incidentally are nothing more than the optical fiber itself). The basic interrogation units are, however, essentially common for each of the three classes of sensor systems. In all cases, some form of the optical domain reflectometer is all that is required. While the above techniques have to date dominated the interest in distributed sensing, we should mention that there are most definitely other options. These typically involve distributed interferometry or polarimetry. There has been some success in monitoring coherent Rayleigh backscatter using a highly coherent source and time gated interferometry. This has already begun to find applications in intruder detection and other security contexts. Similarly, distributed polarimetry, looking for example at changes in local birefringence has been explored in the context of a variety of polarization optical time domain reflectometers (POTDR). Applications have included detecting current carrying conductors at long range using the magnetic field induced changes in local circular birefringence (the Faraday effect) and also in intruder alarms and similar systems. E. Spectroscopy Spectroscopy—the art and science of relating colour measurements in emission, reflection, or transmission to the chemical composition of a sample—is an amazingly versatile tool. <strong>Fiber</strong>-optic spectroscopy utilizes the fiber as the source of illumination to, and the same or a different fiber as a means of collection of light from, a remote sample. Since we are using optical fibers the source of light and the electronic system may be many many kilometers from the sample volume and indeed one source of light may be divided among numerous sample Fig. 17. Optrodes for chemical sensing. points which gives conceptually a huge variety of feasible point sensor network options. There are other operational benefits including the spatial coherence of the optical source fiber, the relatively straightforward mechanical engineering of the source and optical collection geometries and enormous flexibility in the configuration of the sample volume. <strong>Fiber</strong>-optic-based spectroscopy has emerged in two dominant formats. The first looks at broadband illumination and is predominantly associated with measurements made on liquids and solids. The second utilizes very narrow band precisely controlled illumination, typically targeted towards measurements on gases within pressures and temperatures at which line broadening is sufficiently low to preclude significant merging of adjacent absorption lines. The first of these—typically targeting solids and liquids—are frequently referred to as optrodes. 1) Optrode Technologies: Optrodes can make direct measurements on samples or, as shown in Fig. 17, use intermediate chemistry. The latter includes pH indicators, dyes which respond to oxygen hazardous gas species, fluorophores which exhibit quenching in the presence of oxygen and a whole host of other colour change chemical phenomena. Much of the art of spectroscopic measurements lies in deriving appropriate intermediate chemistry and the details of this are covered extensively elsewhere. Direct measurements have been relatively unusual though some considerable success has been achieved (Fig. 18) using combined spectroscopic and scattering measurements to characterize a wide range of liquids. Much of the success of this technique stems from the use of appropriate signal processing based upon pattern recognition to match spectral signatures with reference samples or to group samples within a particular batch (Fig. 19). This approach to the “optical nose” appears to offer much potential for characterization of foodstuffs, oils, other liquid products, and in applications needing precision colour matching. The intermediate chemistry systems have already found numerous niches in water quality monitoring, in some medical applications and sometimes in hazardous gas detection. These are also widely used as biochemical and biomedical probes in for example immunological assays. In contrast to the direct technique, intermediate chemistry always has to rationalize the long term stability of the intermediate chemical compound with sensitivity to the species of interest, with the effects of contamination from the environment with which it must be in contact and also with the impact of inevitable cross sensitivities to other measureands. Consequently,
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Fiber-Optic Sensing: A Historical Perspective - qXwave

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